Since you know the window will never be resized past your minsize, just add something like this (obviously replacing GuiWindowCtrl with the name of your window GUI):
NPC Code:
function GuiWindowCtrl.onResize(nw, nh) {
if (nw == 98) GuiWindowCtrl.width = GuiWindowCtrl.minsize[0];
}
Or to be more safe (in case this strange 98 number is changed):
NPC Code:
function GuiWindowCtrl.onResize(nw, nh) {
if (nw < GuiWindowCtrl.minsize[0]) GuiWindowCtrl.width = GuiWindowCtrl.minsize[0];
}